I thought they had backup generators, etc.. I guess they dont really need them if they don't use them.
From memory, when they had what sounds like exactly the same problem a few months ago it was a case of a power surge from the grid damaged the UPS which in turn took out power. As the generators feed the UPS via the ATS, they were unable to restore power until the damage had been repaired.
It would appear that HE Fremont is a Tier-II facility and so does not have a redundant electrical distribution path to continue to provide power in case the primary distribution path fails or requires maintenance. In a Tier-III facility there would be a failover electrical distribtuion path, often achieved by using a static transfer switch to connect the internal distribution system directly to the raw mains or the generators via the ATS. This can be used to continue to provide power in the event of an issue with the UPS and avoid potentially long periods of downtime.
Normally you would expect there to be a TVSS system between the UPS system and the incoming raw mains feed in order to protect the UPS from potentially damage via surges etc. HE should probably clarify if this is in place and if so, how today's problem still occurred.
All this is based on publically available info about HE, their Fremont facility and the previous outages. I have no insider info, so I could of course be wrong about some details